Creating Windows - Beginning Java - page 835

NOTE A container cannot contain an object of the Window class, or an object of any of the
classes derived from Window . An object of any other class that is derived from Component can
be added to a container.
The components within a container are displayed within the area occupied by the container on the display
screen. A dialog box, for example, might contain a JList<> object offering some options, JCheckbox ob-
jects offering other options, and JButton objects representing buttons enabling the user to end the dialog
or enter the selections — all these components would appear within the boundaries of the dialog box. Of
course, for the contained components to be visible, the container must itself be displayed because the con-
tainer effectively “owns" its components. The container also controls how its embedded components are laid
out by means of an object called a layout manager .
Before I introduce you to what a layout manager is and how the layout of the components in a container
is determined, let's consider the basic methods that the Container class defines that are available to all con-
tainers.
You can find out about the components in a Container object by using the following methods that are
defined in the Container class:
• int getComponentCount() : Returns a count of the number of components contained by the cur-
rent container.
• Component getComponent(int index) : Returns the component identified by the index value.
index is an array index so it must be between 0 and one less than the number of components con-
tained; otherwise, an ArrayIndexOutOfBoundsException is thrown.
• Component[] getComponents() : Returns an array of all the components in the current container.
You can also obtain a reference to a component that contains a given point within the area occupied by
the container by calling the getComponentAt() method, with the x and y coordinates of the point as argu-
ments. If more than one component contains the point, a reference to the component closest to index 0 in the
container is returned.
If you have a Container object, content , perhaps the content pane of a JFrame window, you could iter-
ate through the components in the Container with the following statements:
Component component = null; // Stores a Component
int numComponents = content.getComponentCount(); // Get the count
for(int i = 0 ; i < numComponents ; ++i) {
component = content.getComponent(i); // Get each component
// Do something with component...
}
This retrieves the components in content one at a time in the for loop. Alternatively, you could retrieve
them from the container all at once:
Component[] theComponents = content.getComponents(); // Get all components
for(Component component : theComponents) {
// Do something with component...
}
The getComponent() method returns all the components in the container as an array of elements of type
Component . You can then use the collection-based for loop to iterate over the components in the array.